Risk factor information for severe complications of interpandemic influenza is needed to inform vaccine policy in Thailand. We identified patients with lab-confirmed influenza who were hospitalized with pneumonia during September 2003 to August 2004. Among the 80 case-patients identified through a population-based pneumonia surveillance system in eastern Thailand, cases were 6.2 and 11.1 times more likely to be among persons <1 year old and >75 years old, respectively, compared with the overall population. Cases were also 7.6 times more likely to have chronic respiratory disease. In Thailand, the young, elderly, and those with chronic disease were at high risk for hospitalized pneumonia from influenza.